He is well known for playing Chris Griffin in Seth MacFarlane's Family Guy and for being the co-creator of Cartoon Network's Robot Chicken. His most famous film roles include: My Stepmother is an Alien, Stephan King's: It, Can't Hardly Wait, Idle Hands, Rat Race, Without a Paddle, Italian Job, Scooby Doo 2: Monster's Unleashed, Sex Drive, and Old Dogs. He played Oz in seasons 2 to 4 of Buffy the Vampire Slayer. From 2003-2004, he starred in That 70's Show as Mitch Miller.
As of 2010, Seth is married to fellow actress Clare Grant.